Preheat oven to 145°C with top and bottom heat. Blend the bananas until smooth.
Separate 5 eggs. Add 1 whole egg to the yolks and whisk together.
Add oil, blended bananas and salt to the yolk mixture. Whisk until combined, then add the sifted flour and mix until smooth.
In a separate bowl, whisk egg whites and cream of tartar until foamy. Gradually add the sugar and continue whisking until firm peaks form.
Fold one-third of the meringue into the yolk mixture to lighten it. Pour the yolk mixture back into the remaining meringue and gently fold until fully combined, with no visible white streaks.
Pour the batter into your baking tin. Tap the tin lightly on the counter and run a skewer through the batter to remove any large air bubbles.
Place the tin in a larger tray and fill with hot water to about 1.5cm high.
Bake for 70 minutes.
Remove the cake from the water bath, tap the tin gently on the counter, then invert for 5–10 minutes to minimise shrinkage.
Let cool slightly before slicing and serving.
